Avoid some redundant list allocations

[SimonCropp]

No description provided.

[claude]

Code Review

Overall this is a clean, well-targeted performance improvement. All 18 files follow the same pattern consistently, every downstream consumer correctly accepts IEnumerable<T>, and no correctness issues were introduced.

Checked for: bugs (including multiple enumeration, deferred execution side effects, thread safety), CLAUDE.md compliance (Performance First, AOT compatibility, dual-mode requirements, snapshot testing rules).

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Minor Observation: Double Enumeration in AspireFixture.cs

One removal stands out as slightly counter to the PR's intent. In TUnit.Aspire/AspireFixture.cs, the pending variable is enumerated twice after the .ToList() was removed:

var pending = resourceNames.Where(n => !readySet.Contains(n)); // now lazy

// First enumeration:
logger.LogInformation($"Waiting for {string.Join(", ", pending.Select(n => $"'{n}'"))} ...");

// Second enumeration:
foreach (var name in pending) { ... }

The original .ToList() here was actually intentional — it materializes the filtered set once and re-uses it twice. Without it, the Where predicate runs twice over resourceNames. While this is not a correctness bug (the state doesn't change between the two enumerations, and it's in a timeout/polling code path), it does introduce redundant work rather than eliminating it — the opposite of what the PR aims to do.

A simple fix would be to keep the .ToList() on this specific line, or restructure to iterate pending just once.

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All other changes are genuinely beneficial and correct. Nice cleanup!